#include <stdio.h>

long fact(int n);
long rfact(int n);

int main(void) {
    int num;
    printf("This program calculates factorials.\n");
    printf("Enter a value in the range 0-12 (q to quit):\n");

    while (scanf("%d", &num) == 1) {
        if (num < 0)
            printf("No negative numbers, please.\n");
        else if (num > 12)
            printf("keep input under 13.\n");
        else {
            printf("loop: %d factorial = %ld\n", num, fact(num));
            printf("recursion: %d factorial = %ld\n", num, rfact(num));
        }
        printf("Enter a value in the range 0-12 (q to quit):\n");
    }
    printf("Bye.\n");

    return 0;
}

long fact(int n) {
    long ans;

    for(ans = 1; n > 1; n--)
        ans *= n;

    return ans;
}

long rfact(int n) {
    return n == 0 ? 1 : n * rfact(n - 1);
}
